The invention relates to a method for forming a gap for an erasure head in a video apparatus, an erasure head, and a video recorder having the same.
There has conventionally been known such a configuration for a full-width erasure head used in a video cassette recorder (VCR) etc. that as disclosed in, e.g. Japanese Utility Model Application No. SHO 56-77020, SHO 57-173124, or SHO 63-99307 or Japanese Unexamined Patent Publication No. HEI 7-220229, to form a head gap for that head, two cores made of a magnetic substance sandwich therebetween a sheet made of a non-magnetic substance used as a gap and two sheets of an adhering or welding member sandwiching this non-magnetic sheet so that the cores and the non-magnetic sheet may be adhered or welded with each other via these sheets of adhering or welding members.
Besides the above-mentioned configuration that one sheet of non-magnetic substance and two sheets of adhering or welding members are interposed between two cores, there has been known such a configuration that a melting type non-magnetic substance is melted between two cores and then solidified to provide a gap by itself. Specifically, the gap is formed according to steps shown in FIGS. 10. In the back face of a magnetic block 100 shown in FIG. 10(a), a rectangular escape portion 101 is formed upward from the bottom face of the magnetic substance 100 by cutting and the like as shown in FIG. 10(b). Next, as shown in FIG. 10(c), in the top face of the magnetic substance 100, a groove 102 is formed downward by cutting etc. in a rough C-shape and perpendicular to the extension direction of the escape portion 101, following which, as shown in FIG. 10(d), in the center line of the bottom of the groove 102, a roughly rectangular gap groove 103 with an appropriate width is formed by cutting etc. in the extension direction of the groove 102. Next, as shown in FIG. 10(e), a non-magnetic glass rod 104 is placed on the gap groove 103 and then, as shown in FIG. 10(f), is melted and flowed by an appropriate method into the gap groove 103 and solidified, so that then the bottom surface of the magnetic block 100 is polished to a curved face (R-shape) indicated by a broken line 106, thus finally forming a core 110 having a shape indicated in FIG. 10(g).
However, in a configuration disclosed in Japanese Utility Model Application No. SHO 56-77020, 57-173124, or SHO 63-99307 or Japanese Unexamined Patent Application No. HEI 7-220229, to form a gap in an existing core, it is necessary to use one sheet of a non-magnetic substance and two sheets of an adhering or welding member by a complicated engineering method, thus resulting in poor productivity and expensive manufacturing costs. This in turn increases the costs for manufacturing an erasure head or video recorder using such a core. Moreover, a core formed by a method shown in FIGS. 10 has a small gap width of 50–100 μm and so is difficult to machine and also, to avoid cracking the core when it is formed by heating a core material, e.g. ferrite, and a gap material, e.g. glass, it is necessary to match their linear expansion coefficients properly, which is accompanied by such another problem that the glass melted and flowed into the gap is liable to have an air bubble generated therein and if it occurs in the glass surface, that surface has a minute recess, to which a magnetic particle from a tape is liable to stick when the tape travels in contact with the gap, thus deteriorating the video signal quality and resulting in noise being raised at the recess.